PDS_VERSION_ID                = PDS3
RECORD_TYPE                   = STREAM
OBJECT                        = TEXT
  NOTE                        = "Comments on the MGS Mars Orbiter
    Laser Altimeter (MOLA) PEDR Archive"
  PUBLICATION_DATE            = 1999-10-13
END_OBJECT                    = TEXT
END


COMMENTS ON THE MGS MOLA PEDR ARCHIVE

     This document contains comments and errata concerning the MGS
Mars Orbiter Laser Altimeter (MOLA) Precision Experiment Data Record
(PEDR) Archive.


1.  MOLA PEDR File Names

     The PEDR binary data files on MOLA archive volumes have file
names with the extension ".B", instead of the usual extension of
".DAT" for binary files.

2. Incorrect Values for SPACECRAFT_CLOCK_START_COUNT and
SPACECRAFT_CLOCK_STOP_COUNT in AEDR and PEDR Labels (Orbit Insertion
Volumes Only)

     MOLA volumes MGSL_0001 and MGSL_0002 contain both PEDRs and the
raw data from which they were made, AEDRs (Aggregated Experiment Data
Records). These two volumes contain data acquired during the Orbit
Insertion phase of the mission. On these volumes, the attached PDS
labels for AEDR and PEDR data files contain incorrect values for the
fields SPACECRAFT_CLOCK_START_COUNT and SPACECRAFT_CLOCK_STOP_COUNT,
due to an error in the processing software that created the files.

     The AEDR and PEDR index tables on these volumes were created from
the data file labels, so the index tables also contain the incorrect
values for these two fields.  The error was corrected in data file
labels on subsequent volumes.  The cumulative index tables on
subsequent volumes still contain the incorrect spacecraft clock values
for volumes MGSL_0001 and MGSL_0002.

3.  No Data for Some Orbits (Orbit Insertion Volumes Only)

     PEDR data products do not exist for the pre-mapping phase orbits
373, 375, 413 and 414. The AEDR data for these orbits consist entirely
of noise. During these orbits MOLA observed in an off-nadir position
while the MGS spacecraft was 'tilted', in order to fill in a coverage
gap at the north pole. The range parameters required for these passes
were not transmitted in time due to some miscommunication, and there
were no planetary ranges obtained. Data covering the pole were
acquired successfully on 10 other orbits.

4. No ASCII Text Version of Calibration Document

     The file CALIBDOC.PDF in the DOCUMENT directory is a document
that describes the calibration of MOLA data. It is in Adobe PDF
format.  The document conveys important information in the form of
complex equations, symbols and figures that cannot be represented as
plain ASCII text; hence no ASCII version of this document is provided,
although normally it is required for PDS compliance.

5. Some PEDR Labels from Orbit Insertion have MISSION_PHASE="MAPPING"

     On volume MGSL_2001, the attached PDS labels in PEDR data files
AP01578G.B through AP01682G.B contain the value "MAPPING" for the
MISSION_PHASE keyword, even though the data were acquired during the
Orbit Insertion Phase. The MOLA instrument began acquiring mapping data
at the end of aerobraking, although the Mapping Phase did not officially
begin until about two weeks later.

6. No PDS Labels for Software Files

     Source code and executable files in the SOFTWARE directory and
subdirectories are missing required PDS labels. The labels are
required for information purposes only, and have no effect on the
viability of the software. The software will operate as intended.

7. MOLA Spacecraft Clock Count Values Are Nonstandard

   Spacecraft Clock Count (SCLK) values in MOLA products are recorded
differently from the MGS SPICE standard. The standard specifies that an
SCLK value consists of a string of digits with an embedded decimal
point.  The number to the left of the decimal point represents seconds,
and the number to the right of the decimal point (3 digits) represents a
fraction of a second in increments of 1/256th. For example, an SCLK
value of 627609638.125 means 627609638 seconds and 125/256ths of a
second. The fractional number should always be less than or equal to
255.  

   MOLA SCLK values are recorded slightly differently. The fraction
of a second is the MOLA fine time in ticks divided by 256. The fine time
comes from a MOLA oscillator synchronized to an 8hz interrupt. This
fraction can be a number greater than 255.

   Since the MOLA oscillator tick counter is often in error, the
fractional part of the MOLA SCLK value can be inaccurate.  Therefore
only the whole part of the SCLK value should be used, and then only as
an approximate beginning or ending time for the observation.